Printers are currently found in many forms, however all printers share common characteristics such as a print head, a platen and a control mechanism. The control mechanism controls the motion of the print head relative to the paper, selects a character to be printed, and advances and retracts the paper as necessary.
It is undesirable for a printer to operate without paper. Ink-based printers that are operated without paper will transfer the ink into the platen which may in turn stain the back sides of subsequent sheets of paper and possibly damage print writes in the print head. Thermal printers operated without paper may overheat without paper present to absorb the heat generated by the print head. Also, any printer that operates without paper will cause frustration when documents must be reprinted. Most modern printers, therefore, include a control mechanism to stop the printer if paper runs out and to prevent the printer from starting to print unless an adequate paper supply is present. This control mechanism typically includes a limit or proximity switch to detect the presence or absence of paper.
For example, the apparatus disclosed in U.S. Pat. No. 4,111,565 includes a switch associated with a pivotal sensing member which provides an indication to control circuitry that no paper is present for printing. Not disclosed however, is any apparatus to indicate whether the printer's case is open.
In U.S. Pat. No. 5,396,396 an apparatus is disclosed to indicate opened and closed positions of a cover. Not disclosed, however, is any means of indicating the presence or absence of paper for printing.
Most modern printers are enclosed in covers or cases with hinged or removable sections that open for access to the printing and mechanical areas. These printers are not designed to be operated with the cover open, as the internal mechanism may be at high temperature or include numerous moving parts. This is especially true in the case of a thermal printer where the print head can be damaged if the print head is driven when the cover is open and the print head is not against the platen and nothing is present to absorb heat from the print head. Therefore, these printers typically include an interlock that will prevent the printer from operating with the cover open. The usual interlock includes a limit or proximity switch used to detect whether the printer is open or closed.
The control switches used to detect paper and determine whether the printer is open must be durable and capable of handling a large number of cycles without failure, as failure of these switches can result in damage to the printer or injury to the operator. As a result, the switches used for these tasks are usually expensive. The use of separate switches for these tasks adds substantial expense to the cost of the printer because of associated hardware costs. What is needed, therefore, is a simple, cost-effective method to detect the presence of paper and to determine cover position without duplicating components and increasing the manufacturing costs of the printer.